In a gas turbine of this type, temperatures which may lie in the range of between 1000° C. and 1400° C. occur in the flow duct after it has been acted upon by hot gas. The platform of the turbine blade, as a result of the annular arrangement of a number of such turbine blades of a blade stage, forms part of the flow duct for a working fluid in the form of hot gas which flows through the gas turbine and thereby drives the axial turbine rotor via the turbine blades. Such a high thermal stress on the flow duct boundary formed by the platforms is counteracted in that a platform is cooled from the rear, that is to say from the turbine blade root arranged below the platform. For this purpose, the root and the platform region conventionally have suitable ducting for action by a cooling medium.
An impact-cooling system for a turbine blade of the type initially mentioned may be gathered from DE 2 628 807 A1.
In DE 2 628 807 A1, to cool the platform, a perforated wall element is arranged upstream of that side of the platform which faces away from the hot gas, hence downstream of the platform, that is to say in between a blade root and the platform. Cooling air under the platform, that is to say in between a blade root and the platform. Cooling air under relatively high pressure impinges through the holes of the wall element onto that side of the platform which faces away from the hot gas, with the result that efficient impact cooling is achieved.
EP 1 073 827 B1 discloses a novel way of designing the platform region of cast turbine blades. The platform region is designed as a double platform consisting of two platform walls lying opposite one another. What is achieved thereby is that the platform wall directly exposed to the flow duct and consequently to the hot gas and delimiting the flow duct can be made thin. The design in the form of two platform walls results in a functional separation for the platform walls. The platform wall delimiting the flow duct is responsible essentially for the ducting of the hot gas. The opposite platform wall not acted upon by the hot gas takes over the absorption of the loads originating from the blade leaf. This functional separation allows the platform wall delimiting the flow duct to be made so thin that the ducting of the hot gas is ensured, without substantial loads in this case having to be absorbed.
Normally, in the platform region and in the root region of the blade leaf, between the blade leaf and platform, there is a predetermined relatively high accumulation of material in the case of conventional turbine blades on account of boundary conditions which are due ultimately to production by casting and to strength requirements as a result of the stress undergone by a turbine blade. Such a relatively high accumulation of material at the same time also impedes the outflow of heat from this region by means of cooling methods installed in the blade interior and also prevents the direct cooling of these regions by cooling medium. It is known, in order to cool these regions, to provide film cooling on the outer surface of a blade in the root region of the blade leaf and in the platform region. For this purpose, in the vicinity of these outer surfaces, a cooling film is applied from a corresponding open gap system to these boundaries of the flow duct acted upon by working medium in the form of hot gas. This is, in principle, a functioning solution for cooling the abovementioned platform and root regions with their relatively high accumulations of material. Nevertheless, to achieve this, a considerable amount of cooling air is necessary on account of the complicated secondary flow situation in the flow duct, where vortex configurations of the secondary flow may cause the cooling film to be lifted off and deflected away from said outer surfaces. Consequently, when a gas turbine is operating in reality, this known procedure may lead to an uncooled root region and platform region in these regions where access is difficult. An advantageous configuration of these regions would be desirable.